Mardi Gras is later in 2017 -- Feb. 28 -- than it was in 2016, when Mardi Gras fell on Feb. 9. But in 2018, Mardi Gras is early again, Feb. 13. And in 2019, Mardi Gras is really late -- March 5.
So why isn't Mardi Gras on the same day every year, like Christmas? It's all about Easter, which isn't on the same day every year. Buckle up, because this involves math, astronomy and religion.
Easter falls on the first Sunday following the first ecclesiastical full moon that occurs on or after the day of the vernal equinox, March 21. That means Easter can never occur before March 22 or later than April 25.
What's an ecclesiastical full moon?
The Astronomical Applications Department of the U.S. Naval Observatory explains that "the full moon involved is not the astronomical full moon but an ecclesiastical moon (determined from Catholic Church tables) that keeps, more or less, in step with the astronomical full moon."
The AA explains that "In 1582, Christopher Clavius and a council working at the direction of Gregory XIII (Pope of the Roman Catholic Church) completed a reconstruction of the Julian Calendar producing new Easter tables."
So what does this have to do with Mardi Gras?
Mardi Gras is usually set 47 days before Easter. In any given year, Mardi Gras will fall on any Tuesday between Feb. 3 and March 9. These two dates are extra special, because Mardi Gras will fall on them only once in a lifetime, each occurring roughly once every 100 to 150 years. Read more here.
Mardi Gras is the day before Ash Wednesday, the first day of Lent, which the Catholic Church and some Protestant churches observe leading up to Easter as a time of repentance, marked by fasting, abstinence, prayer and almsgiving.
2018: Feb. 13
2019: March 5
2020: Feb. 25
2021: Feb. 16
2022: March 1
2023: Feb. 21
2024: Feb. 13
2025: March 4
If you really want to plan ahead, and practice your math, here is a list of Easter dates through 2299, courtesy of the Astronomical Society of South Australia